From February 1985 to May 1995, 1589 mechanical valve replacement procedures were done at the Koşuyolu Heart and Research Hospital. In ten year period, 28 mechanic valve dysfunction were detected. There were 20(71.4%) mitral valve dysfunction, 7 (25%) aortic valve dysfunction and 1 (5%) double valve dysfunction. It was realized that 10 patients, were on warfarin sodium treatment as PT value would be 2 times of normal while the other 18 patients stoped the medication. 19 (67.9%) patients had unileaflet prothetic valve and 9 (32.1%) patients had bileaflet prosthetic valve. There were 15 (54%) female and 13 (46%) male patients. The mean age was 42.1 (36-58). 18 cases (64.2) were operated on in the emergent conditions and 3 patients were operated in the semiemergent conditions. During the anatomo-pathologic investigation from resected valve materials, organised thrombus was found in 20 patinets, vegetation with thrombus was found in 7 (25%) patients, 16 (57.2), bileaflet prosthetic valve, 6 (21.%) unileaflet prosthetic valve and 6 (21.4%), bioprostesis weere replaced at the reoperations. IABP was used one patients. Hospital mortality was 14.3 % (4 cases). The late mortality rate was 8.3 % (2 patiens) Up to our experinece, the most important problem in this matter is early diagnosis and urgent surgical therapy.